Field of the Invention
The present invention is directed to a system, method and computer program product that relates to a renewable power production facility, such as a wind turbine generated power production facility that produces electrical power that is applied to a power grid. More specifically, the present invention is directed to systems, methods and computer program product for enhancing the commercial value of electric power produced by wind turbine facilities so as to make that electric power as commercially valuable and fungible as electric power produced by other plants such as fossil fuel power plants, hydroelectric plants, nuclear plants and the like.
Discussion of the Background
Wind power is a “natural” power production source that instinctively should be regarded as an optimum source of energy for producing electric power. Wind power does not require the burning of fossil fuels, does not result in nuclear waste by-products, does not require the channeling of water sources, and does not otherwise disturb the environment. On the other hand, wind power is a variable (stochastic) power generation source, thus not offering power production facilities the type of control that the power production and grid facility would like to have in producing commercially reliable power. To address this variability issue, even the early pioneers of wind power attempted to identify ways to “store” wind generated electric power in times of excess, so as to later compensate for times when there are lulls in the wind. For example, Poul La Cour (1846-1908) from Denmark, was one of the early pioneers in wind generated electricity. Poul La Cour built the world's first electricity generating wind turbine in 1891. This design included DC generators and stored energy as hydrogen. Poul La Cour was concerned with the storage of energy because he used the electricity from his wind turbines for electrolysis in order to produce hydrogen for the gas lights in his school. This concept of energy storage has not been abandoned and even modern inventors of wind turbine electric generation facilities are still trying to identify ways to use physical media to store the energy produced by windmills (see e.g., U.S. Pat. No. 5,225,712, which uses fuel cells, batteries, and the like as physical media to store electrical power). In the early days, wind energy plants were generally isolated from one another and provided small scale generation facilities. Through a variety of experiments wind energy plants have generally evolved and now a common theme is to group a number of wind turbines together so as to form farms that can generate up to tens of megawatts via the aggregation of smaller plants that produce slightly above only one megawatt each. Most modern rotor blades on large wind turbines are made of glass fiber reinforced plastics (GRP). These wind power plants are today planned to grow slightly above three megawatts per unit, limited by a reliable size of the wind turbine, (the “propeller”).
A perplexing task that has somewhat stifled the use of wind power plants is that there has been no commercially viable way, in light of the price of fuel generated by other power plants, to effectively store electricity generated by windmills during periods of peak production, so as to make up for periods when the wind slows. As a consequence, the capital cost, lack of production control, size, and reliability problems limited the proliferation of such wind plants between the periods of 1890 and 1970. As a consequence, the use of wind power declined sharply both with the spread of steam-engines and with the increase in scale of electrical power utilization. Thus, windmills generally were only limited for small scale processes and were unable to compete with large scale steam powered electrical power facilities. Furthermore, the commercial cost of such wind-generated power was much greater compared to those with generating systems based on coal, oil, gas and hydro. Nevertheless, being strong advocate for windmills, Denmark pioneered the effort between the era of 1970 and 1985 to bring back windmill technology in an attempt to make windmill generated electricity a mainstay of modern electric generation plants. To this end, Denmark established some rules regarding grid connections from the windmills, (e.g., Specifications for Connecting Wind Farms to the Transmission Network”, ELTRA I/S ELT 1999-411a., as well as Swedish documents TAMP-1122400 and DAMP-1101300, Sv. Elverksforeningen, the entire contents of which being incorporated herein by reference).
As recognized by the present inventors, there are several drawbacks associated with using wind power systems. First, it should be recognized that there is a strict frequency control on the AC power that is provided to the grid. For example, in the power grid in Europe, the AC frequency is held generally constant at 50 hertz, with an attempt to maintain a maximum frequency variation between plus or minus 0.1 hertz. This means that there must be a continuous balance between the input of energy and the output of electrical power in such an AC system. If consumption is greater than production, the grid frequency drops. If production is greater than consumption, the grid frequency rises. Thus, power companies that provide power to the electric grid must be coordinated so that those adding power are doing so at a time when the demand for that power exists, and also is done in coordination with other providers. While there is a system that is employed to coordinate the activities of different power producers as will be discussed with respect to FIGS. 2-4, the present discussion will now focus on conventional wind turbine electrical power production facilities so as to further explain conventional practice for how to design such facilities.
A number of different options have been attempted to make wind turbine generated power facilities more reliable and predictable, thus “more mainstream” as compared to other power production facilities. In a first typical windmill power generation facility, an asynchronous machine is used that acts as a generator but also inherently consumes reactive power from the AC grid. Consequently, the facility employs a fixed capacitor bank so as to compensate the amount of reactive power that is consumed, thus providing for a more reasonable power factor (cosine of the angle between current and voltage). However, as recognized by the present inventors, there is a risk with such systems, namely where the capacitor bank causes the system to become self-magnetized thus causing the frequency to differ by as much as tens of hertz from the standard oscillation frequency after a fault occurs.
Many wind power plants are erected with a speed adaptation mechanism (usually a gearbox) between the wind turbine and the electric generator so that an AC frequency produced by the wind turbine generator matches that of the power grid. These systems use a mechanical gearbox to increase the speed of the generator shaft. However, the use of this mechanical gearbox increases the cost by three to five times the cost of the generator, also having dramatic increases in the mean time between failure, and mean time to repair of the device, thus not making these designs commercially competitive with the more reliable and less costly fossil fuel power production facilities.
Some windmill-based systems attempt to address power quality aspects at the grid connection, which often manifest themselves as a tower shadow that provides a low-frequency periodic disturbance. This low-frequency periodic disturbance is referred to as “flicker” (e.g. about a 1 hertz variation) that provides for an inconsistent wavering light or power production. These facilities provide static-VAR compensators (SVC) or local energy storage units to provide compensation power.
More elaborate schemes have been developed to make wind-power more competitive with other types of power in the market. Once again the systems are based on the use of energy storage. FIG. 1 is an example of such a system. As seen in FIG. 1, a turbine blade 12 turns at a rate related to wind speed. Some control may be asserted by adjusting the pitch of the blades, as well as by providing an amount of torque adjustment to control the generator by way of generator controllers and active rectifiers. Notably, the system in FIG. can be divided into three components. The first component is between the turbine and the output of the active rectifiers (Rectifier A and Rectifier B). The second component is the DC link between the output of the active rectifiers and the inverters. The last part of the link is between the inverters and the utility grid. The function of the first part of the system (i.e. between the blade 12 and the active rectifier) is to convert the wind into variable speed electrical power, and then rectify that variable frequency AC into a DC voltage. Thus, the output of the first part is a DC voltage that is coupled onto a DC line (see e.g. the line disposed between the active rectifier and the inverter). This DC line then passes this frequency independent electrical power to a location in which an inverter is maintained. At the inverter, an inverter controller 50, 52 is used to produce pulse width modulation (PWM) signals so as to actuate switches within respective inverters thus generating output signals at any particular AC frequency, namely the grid's frequency. A power factor controller may be used to control how the waveform is generated so that the output waveform has a power factor that is consistent with requirements placed on that particular windmill.
Reactive power is important to the operation of an AC power grid. As discussed in U.S. Pat. No. 4,941,079, the contents of which being incorporated herein by reference, some of the advantages are explained. In all AC generator stations of the power utilities, such control is typically achieved through a speed governor and a field excitation regulator. The PWM converter is not encumbered by the long time constants associated with the speed governors and with the generator field inductance. For this reason, the PWM converter is expected to surpass the performance of the AC generator station in providing dynamic enhancement in the utility system. Thus, the general state of the art suggests that the use of power electronics, such as pulse width modulation-based (PWM) converters, provides reactive power control separate from active power control. However, as recognized by the present inventors, rotating electric machines, like generators and compensators, possess not only an ability to control reactive power, but also an overload capability which is superior to all types of power electronics systems, especially PWM IGBT (Insulated Gate Bipolar Transistor), with very limited overload capability. Furthermore, rotating electric machines are able to control the amount of reactive or active power seen from a power source connected to the machine. The primary control of the reactive power is achieved by an automatic voltage regulator (AVR), which controls the magnitude of the output voltage waveform and thereby can control the magnitude of the terminal voltage at the machine. The corresponding control of the active power is achieved by the automatic load-frequency control (ALFC) loop, which uses the frequency as an indirect measure of the active power balance in the grid.
In an improvement to the system shown in FIG. 1, U.S. Pat. No. 5,225,712 describes the use of energy storage devices, based on hydrogen and fuel cells, electrochemical accumulator batteries or the like as a substitute to the capacitors placed on the DC line between the active rectifiers and the inverters shown in FIG. 1. However, as recognized by the present inventors, such devices have a very high cost per kWh compared to the sales price and are sometimes used at the DC voltage link so as to balance power fluctuations as a result of wind gusts and wind lulls.
Recently there have been a number of wind power plants that have been erected at wind farms with constant-speed and/or variable-speed units connected to the same point in the electric power distribution grid. These systems, simplify the power quality issues like the remedial use of static-VAR compensators, discussed above, as well as simplifying maintenance and operation. The present inventors recognize that such connections have not simplified the power grid starting procedures, maintenance, fault handling based on large-short-circuit power, etc. With regard to fault handling, it is noted that grid operators require, desirably, the ability of a power plant to produce high short-circuit power conditions so that there is sufficient electrical current available to trip circuit breakers on the transmission grid, should a fault be detected. One of the problems with conventional wind power plants is that they do not possess this capability, thereby creating a potential hazard for devices that are connected to the grid.
The discussion up to this point has been focused on different techniques that have been attempted in wind power plant facilities to adapt the electricity generated from wind power to make the power suitable for application onto transnational, national, or regional power grids. However, as recognized by the present inventors, there is yet another shortcoming besides simply the application of the power to the power grids, namely the commercial viability and scalability of the electricity generated from wind power as an economic competitor with other types of electric power. In order to appreciate the limitations with wind generated electric power, a discussion of how other types of power is handled is in order. The present discussion will be directed primarily to that in Scandinavian countries, although it is equally applicable in other countries and regions where electric power deregulation has been instituted. Many of these topics are addressed in “The Swedish Electricity Market and the Role of Svenska Kraftnat”, published by Svenska Kraftnat, the National Swedish Grid Company, 1999, available at www.svk.se.
As seen in FIG. 2, electricity producers generate power and feed it into a network, either a national grid, regional network or local network. Network owners are responsible for transmitting the electrical power from the producer to the consumer. Consumers, which include everything from industries to households, take electricity from the electricity networks and consume it. Each consumer must have an agreement with an electricity trader to be able to buy electricity. The power trading company is in contact with its consumers and sells electricity to them. The power trader can have the role of electricity supplier and/or balance provider, both roles can exist within the same or different companies. The electricity supplier has the supply agreement with the consumer. The balance provider is financially responsible for the electricity that the trader sells always being in a state of balance with the electricity purchased so as to cover consumption. The balance providers provide “fine tuning” needed so as to make sure that the amount of power provided to the network matches the particular load at any given time, otherwise the grid frequency will vary. There are organized marketplaces, such as for example, power exchange Nord Pool, as well as brokers, that make standard agreements that make it easier for the participants in the power market to do their business with one another. The bulk of the trade in electricity on the market takes place via bilateral agreements between electricity producers and electricity traders.
FIG. 3 shows the contract network and daily flow of information between participants in the electricity market, which in the present example is Sweden. Grid customers (about 30) include electricity producers and regional network operators. Balance providers (about 50) are electricity suppliers that provide information regarding their operations to the balance authority and system operator. Included in this information is market information provided by the Nord Pool trading center, which is also exchanged between the balance providers and Nord Pool itself. The system operator also has balance obligation agreement settlement information which is exchanged between the balance provider and system operators. Based on system operator instruction, the balance providers provide up-to-date control over the amount of electrical energy (characterized in a short fall or surplus), that is applied to the grid based on load variations and other contracts that have been executed for power delivery to the grid. Furthermore, network owners total-up the measured production and consumption values each hour on their networks as well as for the balance providers that exist on the networks. The totals are then reported to the system operator as a balance settlement and to the balance providers.
As is clear from the detailed communications that exist between the different entities in FIG. 3, the operation of the grid must be planned. As a consequence, the system operator requires that balance providers submit under a balance obligation agreement, different required information. Among other things, this information includes production plans and load forecasts every evening prior to the coming delivery day, and when required update this information on a continuing basis. Using this data then, the system operator can estimate the load and assess whether bottlenecks may arise on the network. The system operator is also in regular contact with the control centers of electricity producers, regional and local network owners and system operators of the other Nordic countries. In order to coordinate information, the different system operators have agreed to distribute important information about the grid and balance services via Nord Pool's website, www.nordpool.com. This information includes historical information regarding the total reported production per country per hour, the total calculated consumption per country per hour, measured power exchanges between countries' systems per hour, available transmission capacity per hour, price and volume of trade and regulating power per country and per hour, as well as plans and information in real time, which includes network disruptions that have occurred which are of significance to the market, and other types of faults.
With regard to most of the power delivery, electricity power options are traded as part of a Nordic power exchange futures market. The combined use of electric power options and forward and future power contracts offers greater opportunity for spreading and handling of risk in power trading. A notable feature in how trading is performed, is that Nord Pool's electric power options are standardized and thus carry a number of fixed terms and conditions. For example, the forward contracts are based on two seasonal contracts and two year contracts. A new series is listed on the first trading day of the exercise day of the previous contract series. The exercise day is the third Thursday of the month before the first delivery month of the underlying instrument. Details of how the power exchange is performed is described in the document “Eloption”, May 1, 1999, available from www.nordpool.com, the entire contents of which being incorporated herein by reference.
What is notable however, as recognized by the present inventors, is that electricity from wind power, and the limitation within a wind-variable system, is not well suited with the current state-of-the-art systems for providing power to the power grid. For example, the risk is high to a wind turbine provider for entering into a forward contract, given the stochastic nature of wind power, and thus the stochastic nature of a wind turbine as a power generation source, that could be expected to be generated by that provider at the time of delivery. While wind powered systems that employ physical assets as part of the system for providing actual energy storage present one potential solution. The inherent expense of such systems makes the opportunity to offer power during periods of low wind speed very expensive since the wind power operator needs to purchase the physical assets for storing the electrical power.
Aside from providing long term planning, there is also short-term balance requirements that may be placed on system operators for filling gaps or short falls in expected power demands or load variations. A time table for trading imbalance is shown in FIG. 4 which describes the dynamic nature of how balance regulation is performed. Balance providers and other participants can trade in electricity in order to plan their physical balances right up until just before delivery hour. By physical balance, it is meant that the production and purchasing are in balance with consumption and sale. Trading can take place on the spot market of the power exchange Nord Pool, which closes at noon the day before delivery. Alternatively, trading in electricity can take place on the adjustment market of the EL-EX power exchange from 3:00 on the day before up until two hours prior to delivery, or bilaterally. The system operator and balance regulator, regularly accepts bids (volume in power in MW) from producers who are willing to quickly (within 10 minutes at the outside) increase or decrease their level of production. Consumers, too, can submit bids for increasing or decreasing their level of consumption (known as load shedding). Balance settlement is performed at noon the day after delivery.
As recognized by the present inventors, a limitation with conventional wind power systems is that unless there is some physical media for storing the electrical power at the local generation facility, conventional systems cannot reliably perform in either the balance regulation or the longer term Nord Pool exchange, due to variability of the wind power. This concept is reflected in the article by Lennart Söder “The Operation Value of Wind Power in the Deregulated Swedish Market”, Royal Institute of Technology, Sweden, Nordic Wind Power Conference 13-14, March 2000, page 5, paragraph 4.1.3, where it is explained that for wind power the construction of the exchange makes it difficult to put bids. The bids on Nord Pool have to be put 12 to 36 hours in advance of real delivery. Lennart Söder states that this makes it in reality nearly impossible to trade wind power bids since the forecasts normally are too bad for this time. Thus, wind power is generally recognized as a environmentally friendly type of power, however not as commercially valuable or fungible as other types of electricity such as that generated by fossil fuels.
To further emphasize this point, an article by Ackermann, T., et al. “Wind Energy Technology and Current Status: A Review”, Renewable and Sustainable Energy Reviews, Paragammon Press, April 2000, pages 317-366, the entire contents of which being incorporated herein by reference, shows in FIG. 8 thereof (page 347) the probability of a change in power output as a percent of installed capacity. This analysis shows that with a probability of 30% the hourly mean wind power output from one hour to the next would be plus or minus 1% of the installed capacity, plus or minus 4% from one four hourly mean to the next and plus or minus 12% between the 12 hourly means. The largest change in power output to be expected between hourly mean power output values is about 40% of installed capacity. Long-term variations in wind speed, between one year and the next are usually quite low, as observed in this study. Thus, while short-term variations (within the 12-hour period) may be substantial, over the long haul (a year or more), the data appears to indicate that relatively small annual variations will occur. This is recognized by the present inventors as an issue of predictability, which would make wind power a viable asset in the Nord Pool exchanges provided there is a cost effective mechanism for storing energy that may later be released on demand to generate electrical power.